Temperature insensitive external cavity lasers on silicon

ABSTRACT

A technique related to a semiconductor chip is provided. An optical gain chip is attached to a semiconductor substrate. An integrated photonic circuit is on the semiconductor substrate, and the optical gain chip is optically coupled to the integrated photonic circuit thereby forming a laser cavity. The integrated photonic circuit includes an active intra-cavity thermo-optic optical phase tuner element, an intra-cavity optical band-pass filter, and an output coupler band-reflect optical grating filter with passive phase compensation. The active intra-cavity thermo-optic optical phase tuner element, the intra-cavity optical band-pass filter, and the output coupler band-reflect optical grating filter with passive phase compensation are optically coupled together.

BACKGROUND

The present invention relates to an external cavity laser on silicon,and more specifically, to temperature insensitive external cavity laserson silicon.

An optical cavity or optical resonator is an arrangement of mirrors thatforms a standing wave cavity resonator for light waves. Optical cavitiesare a major component of lasers, surrounding the gain medium andproviding feedback of the laser light. They are also used in opticalparametric oscillators and some interferometers. Light confined in thecavity reflects multiple times, producing standing waves for certainresonance frequencies. The standing wave patterns produced are calledmodes. Longitudinal modes differ only in frequency while transversemodes differ for different frequencies and have different intensitypatterns across the cross section of the beam.

Different resonator types are distinguished by the focal lengths of thetwo mirrors and the distance between them. Flat mirrors are not oftenused because of the difficulty of aligning them to the needed precision.The geometry (resonator type) must be chosen so that the beam remainsstable, which means that the size of the beam does not continually growwith multiple reflections. Resonator types are also designed to meetother criteria such as minimum beam waist or having no focal pointinside the cavity. Optical cavities are designed to have a large Qfactor, which means that the light beam will reflect a very large numberof times with little attenuation. Therefore, the frequency line width ofthe beam is very small compared to the frequency of the laser.

Light confined in a resonator will reflect multiple times from themirrors, and due to the effects of interference, only certain patternsand frequencies of radiation will be sustained by the resonator, withthe others being suppressed by destructive interference. In general,radiation patterns which are reproduced on every round-trip of the lightthrough the resonator are the most stable, and these are the eigenmodes,known as the modes, of the resonator.

Resonator modes can be divided into two types: longitudinal modes, whichdiffer in frequency from each other; and transverse modes, which maydiffer in both frequency and the intensity pattern of the light. Thebasic or fundamental transverse mode of a resonator is a Gaussian beam.

The most common types of optical cavities consist of two facing plane(flat) or spherical mirrors. The simplest of these is the plane-parallelor Fabry-Pérot cavity, consisting of two opposing flat mirrors.Plane-parallel resonators are therefore commonly used in microchiplasers, microcavity lasers, and semiconductor lasers. In these cases,rather than using separate mirrors, a reflective optical coating may bedirectly applied to the laser medium itself.

SUMMARY

According to one embodiment, a semiconductor chip is provided. The chipincludes an optical gain chip attached to a semiconductor substrate andan integrated photonic circuit on the semiconductor substrate. Theoptical gain chip is optically coupled to the integrated photoniccircuit thereby forming a laser cavity. The integrated photonic circuitincludes an active intra-cavity thermo-optic optical phase tunerelement, an intra-cavity optical band-pass filter, and an output couplerband-reflect optical grating filter with passive phase compensation. Theactive intra-cavity thermo-optic optical phase tuner element, theintra-cavity optical band-pass filter, and the output couplerband-reflect optical grating filter with passive phase compensation areoptically coupled together.

According to one embodiment, a method of configuring a semiconductorchip is provided. The method includes providing an optical gain chipattached to a semiconductor substrate, and providing an integratedphotonic circuit on the semiconductor substrate. The optical gain chipis optically coupled to the integrated photonic circuit thereby forminga laser cavity. The integrated photonic circuit includes an activeintra-cavity thermo-optic optical phase tuner element, an intra-cavityoptical band-pass filter, and an output coupler band-reflect opticalgrating filter with passive phase compensation. The active intra-cavitythermo-optic optical phase tuner element, the intra-cavity opticalband-pass filter, and output coupler band-reflect optical grating filterwith passive phase compensation are optically coupled together.

According to one embodiment, a semiconductor chip is provided. Thesemiconductor chip includes an optical gain chip attached to asemiconductor substrate, an N-port demultiplexing filter, and aplurality of integrated photonic circuits on the semiconductorsubstrate. The optical gain chip is optically coupled to the pluralityof integrated photonic circuits thereby forming a laser cavity. Theplurality of integrated photonic circuits each comprise an outputcoupler band-reflect optical grating filter with passive phasecompensation, an active intra-cavity thermo-optic optical phase tunerelement, and an intra-cavity optical band-pass filter. The outputcoupler band-reflect optical grating filter with passive phasecompensation, the active intra-cavity thermo-optic optical phase tunerelement, the intra-cavity optical band-pass filter are optically coupledtogether. The N-port demultiplexing filter is configured to providedifferent wavelengths of light to individual ones of the plurality ofintegrated photonic circuits.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention. For a better understanding of the invention with theadvantages and the features, refer to the description and to thedrawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The forgoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1 illustrates a single frequency cavity diagram of a laser on asilicon chip according to an embodiment;

FIG. 2 illustrates a single frequency cavity diagram of a laser on asilicon chip according to an embodiment;

FIG. 3A illustrates a graph showing how the phase changes and passivephase compensation temperature changes correspondingly, according to anembodiment;

FIG. 3B illustrates a schematic of the passive intra-cavitytransmission-mode optical phase compensation element with detailsaccording to and embodiment;

FIG. 4 illustrates an example intra-cavity transmission-mode opticalband-pass filter according to an embodiment;

FIG. 5A illustrates an implementation of an active intra-cavitytransmission-mode thermo-optic optical phase tuner element as abroadband thermo-optic tuner according to an embodiment;

FIG. 5B illustrates another implementation of the active intra-cavitytransmission-mode thermo-optic optical phase tuner element as narrowbandthermo-optic tuners according to an embodiment;

FIG. 6 illustrates an example output coupler band-reflect gratingoptical filter according to an embodiment;

FIG. 7 illustrates an example of a mode converter according to anembodiment;

FIG. 8 illustrates an example implementation of a power monitoraccording to an embodiment;

FIG. 9 illustrates a multi-frequency diagram of a laser on the siliconchip according to an embodiment;

FIG. 10 illustrates another multi-frequency diagram of a laser on thesilicon chip according to an embodiment; and

FIG. 11 illustrates a method of configuring the semiconductor chipaccording to an embodiment.

DETAILED DESCRIPTION

Although laser light is perhaps the purest form of light, it is not of asingle, pure frequency or wavelength. All lasers produce light over somenatural bandwidth or range of frequencies. A laser's bandwidth ofoperation is determined primarily by the gain medium from which thelaser is constructed and by the range of frequencies over which a lasermay operate (known as the gain bandwidth).

The second factor to determine a laser's emission frequencies is theoptical cavity (or resonant cavity) of the laser. In the simplest case,this consists of two plane (flat) mirrors facing each other, surroundingthe gain medium of the laser (again this arrangement is known as aFabry-Pérot cavity). Since light is a wave, when bouncing between themirrors of the cavity, the light will constructively and destructivelyinterfere with itself, leading to the formation of standing waves ormodes between the mirrors. These standing waves form a discrete set offrequencies, known as the longitudinal modes of the cavity. These modesare the only frequencies of light which are self-regenerating andallowed to oscillate by the resonant cavity, while all other frequenciesof light are suppressed by destructive interference. For a simpleplane-mirror cavity, the allowed modes are those for which theseparation distance of the mirrors L is an exact multiple of half thewavelength of the light λ, such that L=qλ/2, where q is an integer knownas the mode order.

In a simple laser, each of these modes oscillates independently, with nofixed relationship between each other, in essence like a set ofindependent lasers all emitting light at slightly different frequencies.The individual phase of the light waves in each mode is not fixed andmay vary randomly due to such things as thermal changes (i.e.,temperature) in materials of the laser. In lasers with only a fewoscillating modes, interference between the modes can cause beatingeffects in the laser output, leading to fluctuations in intensity. Inlasers with many thousands of modes, these interference effects tend toaverage to a near-constant output intensity.

Embodiments are configured to provide temperature insensitive (i.e.,thermal insensitive) optical laser cavities. According to embodiments,the temperature insensitive external cavity lasers on silicon providevarious benefits:

(1) Cost reduction is achievable by simplifying laser fabrication andeliminating operating wavelength tolerance as yield limitation. Lasingfrequency is set by silicon fabricated components that have a highfabrication precision and inherent tunability with no extra cost.

(2) Stabilization of lasing frequency is achievable by active or passivemeans in the silicon fabricated section at any desired temperaturethroughout operation range.

(3) Relative intensity noise (RIN) may be reduced (performanceimprovement) by the cavity length increase and high-extinctionintra-cavity optical filter.

(4) Narrowband filter used in the passive cavity may enable siliconon-chip isolator through time gating modulators at transceiver bitrate.

(5) The III-V chip is identical to plan-of-record distributed feedback(DFB) lasers except that the grating fabrication step is omitted by thelaser vendor.

Now turning to the figures, FIG. 1 illustrates a single frequency cavitydiagram of a laser on a silicon chip 100 according to an embodiment. Thesilicon chip 100 is a laser or laser system. Although silicon maydiscussed as an example chip and substrate material, it is understoodthat other semiconductor materials may be utilized including a germaniumwafer.

The silicon chip 100 has a III-V chip 10 mounted on the substrate 30(e.g., silicon wafer) of the silicon chip 100. The III-V chip 10 mayalso be referred to as a III-V die, a III-V semiconductor chip, and/anoptical gain chip/medium as understood by one skilled in the art. Thecombination of the III-V chip 10 mounted on the silicon substrate 30 ofthe silicon chip 100 may be referred to as a hybrid silicon laser. Thehybrid silicon laser is a semiconductor laser fabricated from bothsilicon and group III-V semiconductor materials. Group III and group Vare designations on the periodic table. The hybrid approach takesadvantage of the light-emitting properties of III-V semiconductormaterials combined with the process maturity of silicon to fabricateelectrically driven lasers on a silicon wafer that can be integratedwith other silicon photonic devices.

The III-V chip 10 may be a laser diode that is an electrically pumpedsemiconductor laser in which the active (gain) medium is formed by a p-njunction (p-type doped region and n-type doped region) of asemiconductor diode similar to that found in a light-emitting diode. Alaser diode is electrically a PIN diode (also referred to as a p-i-ndiode), which is a diode with a wide, undoped intrinsic (I)semiconductor region between a p-type (P) semiconductor and an n-type(N) semiconductor region. The p-type and n-type regions are typicallyheavily doped because they are used for ohmic contacts. The active(gain) region of the laser diode is in the intrinsic (I) region, and thecarriers (i.e., electrons and holes) are pumped into intrinsic (I)region from the N and P regions respectively. While initial diode laserresearch was conducted on simple P-N diodes, modern lasers use thedouble-heterostructure implementation, where the carriers and thephotons are confined in order to maximize their chances forrecombination and light generation. Unlike a regular diode used inelectronics, the goal for a laser diode is that all carriers recombinein the I region and thus produce light. Accordingly, laser diodes arefabricated using direct bandgap semiconductors. The laser diodeepitaxial structure is grown using one of the crystal growth techniques,usually starting from an N doped substrate, and growing the I dopedactive layer, followed by the P doped cladding, and a contact layer. Theactive layer most often consists of quantum wells, which provide lowerthreshold current and higher efficiency. A method of powering some laserdiode is the use of optical pumping. Optically pumped semiconductorlasers (OPSL) use the III-V semiconductor chip 10 as the gain medium,and use another laser (often another diode laser) as the pump source.One skilled in the art understands the use and operation of a laserusing a III-V semiconductor chip.

Referring back to FIG. 1, the III-V chip 10 has a high rear reflective(HR) coating facet 12 on one end and has antireflective (AR) coatingfacet 14 on the other end. The light increases in intensity in the gainregion (intrinsic region (I)) of the III-V chip 10.

The III-V chip 10 is attached/mounted to silicon chip 100 and alignedfor optical coupling by any flip-chip or wirebond mounting option knownto one skilled in the art. The III-V chip 10 (e.g., a hybrid siliconlaser) is an optical source that is fabricated from both silicon andgroup III-V semiconductor materials, where the group III-V semiconductormaterials may include, e.g., Indium (III) phosphide (V), gallium (III)arsenide (V), nitrogen (V), etc. A mode converter 16 is coupled to theIII-V chip 10. In one case, the mode converter 16 may be identical tothat required to couple a distributed feedback laser (DFB) with similarrequirements for low insertion loss and reflection as understood by oneskilled in the art. A mode converter 16 (also referred to as mode sizeconverter) includes optical devices which allow for efficient couplingbetween modes of different sizes. A mode (size) converter (or mode sizeadapter) is an optical device which can be used for expanding orcontracting a mode in the transverse spatial dimensions. For example, amode converter could expand the very tiny mode of the waveguide in alaser diode to a size which fits to the mode of an optical fiber.

The mode converter 16 couples the III-V chip 10 to the waveguide 20.Although shown schematically as entirely external to the III-V chip 10,it is understood that mode converting components 16 may also includecomponents, e.g. active or passive waveguide sections with differentdimensions that the primary gain waveguide section, fabricated on theIII-V chip 10. The waveguide 20 connects to various intra-cavity opticalelements 120, 130, and 140 on the silicon chip 100 as understood by oneskilled in the art. The order of the intra-cavity optical elements 120and 130 shown is arbitrary and is not meant to be limiting. It iscontemplated that either order of the intra-cavity optical elements 120and 130 is possible. The intra-cavity optical elements 120, 130, 140 maybe an external integrated photonic circuit 25 fabricated on the siliconsubstrate 30 of the silicon chip 100.

A laser cavity 24 is formed between the III-V gain chip 10 and theexternal integrated photonic circuit 25, specifically between the HRfacet 12 and the band-reflect grating 140. To provide a basis for thefollowing discussion, the magnitude of the dominant polarization of theelectric field, E or modal amplitude, in the laser resonator will bedescribed as a function of time, t, and longitudinal position, z. Thecoordinate system is defined such that the HR facet (element 12) of theIII-V chip (element 10) is z=0. The expression for the modal amplitudecan then be described by the function E(ω, z, t, T)=A_(forward)(z)·e^(i(ωt-k(ω, z, T)z)+A_(reverse)(z)·e^(i(k(ω, z, T)z-ωt)). The realvalued A_(forward)(z) and A_(reverse)(Z) functions define the amplitudeof the forward and backward propagating fields in the laser cavitysubject to the loss and gain from the intra-cavity elements. Remainingvariables are defined as follows: ω is the angular frequency of theoptical mode of interest; T is the local temperature (i.e. rigorouslyT(z)); k(ω, z, T) is the wavevector of the optical mode of the givenangular frequency, for the given longitudinal position and temperature.For clarity, the effects of reflections from intra-cavity elements areneglected and details associated with the phase change resulting fromtransmission through the intra-cavity filter are neglected. Allintra-cavity elements are treated as waveguides with arbitrary k(z, ω,T) characteristics.

The laser cavity as defined then supports a continuum of longitudinaloptical modes ω₀, ω₁, . . . ω_(m) that are determined by the round-tripconstructive interference condition of the resonator. As is well knownwithin the field, this interference condition is satisfied when theaccumulated optical phase of the round-trip propagation, φ, equals aninteger multiple of 2π. Using the above conventions and defining theposition of effective reflection within the band-reflect grating 140 forthe given modal angular frequency and local temperature as z″(ω,T), theround trip phase is given by:φ(ω,T)=2∫₀ ^(z″(ω,T)) k(ω,z,T)zdz

To simplify the analysis, the case of uniform, frequency independentmodal effective indices, n_(III-V)(T) and n_(Si)(T), will be consideredfor the III-V chip 10 and the silicon external cavity 25 respectively.The longitudinal coordinate for the interface between the III-V chip 10and the silicon external cavity 25 is then defined as z′ with thelengths of the two cavity halves as L_(III-V) and L_(Si)(T)respectively. The length of the silicon external cavity 25 is stillconsidered temperature dependent in this analysis due to z″(ω,T), butthe frequency dependence is ignored. The resulting round trip phase canthen be expressed simply by expanding the wavevector in terms of theeffective index, angular frequency and vacuum speed of light, c, as afunction of position:φ(ω,T)=2ω/c(n _(III-V)(T)L _(III-V) +n _(Si)(T)L _(Si)(T))

Enforcing the phase matching condition, the angular frequency ofoperating mode ω_(m) can then be expressed as:

${\omega_{m}(T)} = \frac{( {m + 1} )\pi\; c}{{{n_{{III} - V}(T)}L_{{III} - V}} + {{n_{Si}(T)}{L_{Si}(T)}}}$

Now, further details of the intra-cavity optical elements 120, 130, 140in the external integrated photonic circuit 25 are discussed below inthe context of the above influence on ω_(m)(T).

The intra-cavity optical element 120 is an intra-cavitytransmission-mode optical band-pass filter 120 with a full-widthhalf-maximum (FWHM) equal to or less than the free-spectral range of thecavity Fabry-Perot (F-P) resonances. The purpose of this filter is toprovide operating longitudinal mode selection through lossdiscrimination such that the filter resonant frequency, ω_(f), isactively tuned to be centered on the desired ω_(m) while providingsufficient round-trip cavity loss discrimination for adjacentlongitudinal modes ω_(m−1) and ω_(m+1) to prevent undesired modes fromreaching lasing threshold and provide a sufficient side mode suppressionratio. The narrow bandwidth of the intra-cavity filter results in areduction of laser output power proportional to the magnitude ofdifference, Δω=|ω_(f)−ω_(m)|. This enables the output power of the laserto be monitored as a feedback parameter for matching the intra-cavityfilter resonance frequency with the longitudinal operating mode throughthe active control of either mode.

The resonance frequency of the intra-cavity transmission-mode opticalband-pass filter 120 is then held constant throughout laser operation(except startup initialization or where intentionally modulated) as afunction of temperature through either an athermal design or activecontrol. Examples of athermal design for the band-pass filter includemodal thermo-optic coefficient compensation by varying waveguide widthsand lengths in a silicon/silicon dioxide interferometer or introducingnegative thermo-optic material cladding such as TiO₂ over a siliconnanowire ring resonator filter. Examples of active control includecontrolling integrated heater power based on a temperature sensorfeedback signal.

In the case of low free-spectral range filters such as ring resonators,the intra-cavity transmission-mode optical band-pass filter 120 mustalso be designed such that the free-spectral range is greater than halfof the FWHM reflection bandwidth of the band-reflect grating 140. Thiscondition ensures that other longitudinal mode orders of the band-passfilter do not provide alternate low round-trip loss longitudinal lasercavity operating modes.

The intra-cavity optical element 130 is an active intra-cavitytransmission-mode thermo-optic optical phase tuner element 130. Theactive intra-cavity transmission-mode thermo-optic optical phase tunerelement 130 may include either a broad-band waveguide section or anarrowband filter such as one or more ring resonator filters in anall-pass transmission phase control configuration. The activeintra-cavity transmission-mode thermo-optic optical phase tuner element130 is configured to adjust round-trip cavity phase to a constant valuewithin the compensated temperature range of laser operation, e.g. 0°Celsius (C)-85° C., based on the measured value of suitable feedbackparameter such as laser output power. The active intra-cavitytransmission-mode thermo-optic optical phase tuner element 130 providesactive control of the round trip phase of the laser operating mode, φ,and this means that the active intra-cavity transmission-modethermo-optic optical phase tuner element 130 requires power to controlthe phase. In the context of previous discussion of round trip cavityphase, the active phase tuner 130 of given length, L_(tune), powered toan elevated temperature, ΔT_(tune), over the ambient temperature T_(amb)controls the operating mode frequency to a constant value, ω_(m)′, thatis independent of T_(amb) by adjusting ΔT through the effectivethermo-optic coefficient of the tuner,

$\frac{d\; n_{tune}}{d\; T}:$

$\omega_{m}^{\prime} = {\frac{( {m + 1} )\pi\; c}{{{n_{{III} - V}( T_{amb} )}L_{{III} - V}} + {{n_{Si}( T_{amb} )}{L_{Si}( T_{amb} )}} + {\Delta\; T_{tune}\frac{d\; n_{tune}}{d\; T}L_{tune}}}.}$

The intra-cavity optical element 140 is a band-reflect grating withpassive phase compensation that is the laser output coupler whilereducing the net round trip phase change as a function of temperature.The relevant design range for the in-band reflectance is between 5% and80%. As discussed for element 120, full-width half-maximum (FWHM)reflectance bandwidth of the band-reflect grating with passive phasecompensation 140 must be less than double the free-spectral range of theband-pass optical filter 120.

The passive phase compensation properties of the band-reflect grating140 is accomplished by designing the temperature dependence of theeffective mirror position, z″(ω,T), to result in a shorter effectivesilicon cavity length, L_(Si)(T), with increasing temperature tocompensate for the positive effective thermos-optic coefficients of theIII-V and silicon waveguides,

$\frac{d\; n_{{III} - V}}{d\; T}$and

$\frac{d\; n_{Si}}{d\; T}$respectively. The desired design condition is then:

${{- \frac{d\; L_{Si}}{d\; T}}n_{Si}} \approx {{\frac{d\; n_{Si}}{d\; T}L_{Si}} + {\frac{d\; n_{{III} - V}}{d\; T}L_{{III} - V}}}$

This design methodology bounds the round trip phase of the desired laserlongitudinal operating mode to within a small total phase change rangefor a specific designed operating temperature range. Continuoussingle-mode operation requires that the remaining round trip phasechange is within the control range of the active intra-cavitytransmission-mode thermo-optical phase tuner 130, e.g. 4π, over thecompensated temperature range of laser operation, e.g. 0° C.-85° C.Assuming that the cavity round trip phase change is monotonic withtemperature, the example case can then be expressed as:|φ(ω_(m),85° C.)−φ(ω_(m)0° C.)|<4π

The design of the passive phase compensation can be understood throughthe temperature dependence of the grating's effective mirror positionand therefore L_(Si) z″(ω_(m),T). For a uniform grating, the effectivemirror position from the input of the grating, L_(eff)(ω,T), can bewritten in terms of the coupling strength, κ(ω,T), and total gratinglength L_(g) as:

${L_{eff}( {\omega,T} )} = {\frac{1}{2\;{\kappa( {\omega,T} )}}{\tanh\lbrack {{\kappa( {\omega,T} )}L_{g}} \rbrack}}$

For the simplest form of thermal compensation, the coupling strengthtemperature dependence can κ(ω,T) maximized such that the effectivegrating length is reduced to compensate for the positive thermo-opticcoefficient of the rest of the laser cavity. This level of compensationmay be sufficient for short laser cavities with strongly reflectinggratings.

Stronger compensation of the net cavity thermo-optic coefficient can beachieved with properly designed chirped gratings. In a chirped grating,the effective index, n, and the grating pitch, Λ, can be varied as afunction of position. For a given frequency, ω, the grating pitch thatresults in maximum reflectance, Λ^(max)(ω,n), is approximately:

${\Lambda^{\max}( {\omega,n} )} = \frac{\pi\; c}{\omega \cdot n}$

The design of a linear chirped grating for passive phase compensation ofthe round trip cavity phase is considered in the context of the previousvariable definitions. The effective index in the grating will beapproximated as constant and equal to the unpeturbed silicon cavity,n_(Si)(T). The grating pitch as a function of position will then bewritten in terms of a chirp rate, dΛ/dz, and central pitch correspondingto the maximum reflectance condition for the nominal operating mode,ω_(m), at reference temperature T₀:

${\Lambda(z)} = {{\frac{d\;\Lambda}{d\; z}( {z - \frac{L_{g}}{2}} )} + {\Lambda^{\max}( {\omega_{m},{n_{Si}( T_{0} )}} )}}$

To simplify the analysis, we can treat the effective mirror position asbeing defined as the point where the grating pitch maximizes thereflectance for the operating mode angular frequency ω_(m) attemperature T. We are then interested in obtaining the resulting changein silicon cavity length, L_(Si), with temperature that this effect canprovide. Substituting variables from the previous equations, taking thederivative with respect to temperature and neglecting higher orderterms, we can obtain the following relation:

$\frac{d\; L_{Si}}{d\; T} \approx {{- ( \frac{\pi\; c}{\omega_{m}{n_{Si}^{2}( T_{0} )}} )}\frac{\frac{d\; n_{Si}}{d\; T}}{\frac{d\;\Lambda}{d\; z}}}$

Utilizing the previous design criteria for the passive thermo-opticphase compensation criteria to enable thermally-insensitive laseroperation, the required grating chirp parameter can then be approximatedas:

$\frac{d\;\Lambda}{d\; z} \approx \frac{( \frac{\pi\; c}{\omega_{m}{n_{Si}( T_{0} )}} )\frac{d\; n_{Si}}{d\; T}}{{L_{S\; i}\frac{d\; n_{Si}}{d\; T}} + {L_{{III} - V}\frac{d\; n_{{III} - V}}{d\; T}}}$

This approximate chirp parameter is derived and provided to provide aconcrete design example but is not the rigorous criteria for thedisclosed laser cavities. Both the coupling and effective indextemperature dependences must be considered to choose the correct chirpparameter. Generally, the required chirp parameter results in a“red-chirped” grating such that dΛ/dz is a positive value. It should benoted that this criteria is opposite to traditional external cavitychirped grating designs that choose a negative dΛ/dz to improve noisecharacteristics. The disadvantage of a positive dΛ/dz chirp design inthis configuration is mitigated by the large longitudinal laser cavitymode free spectral ranges enabled by the compact integrated cavitydesign.

The laser beam (output) of the laser system on the silicon chip 100 ismonitored by a power monitor 18. The power monitor 18 is coupled to thewaveguide 20. Power monitoring in the laser system on silicon chip 100is utilized for control of the intra-cavity phase, to maintain efficientsingle-mode operation, for error-free link operation, and for using thelaser across the operating temperature (e.g., 0-85° C.). The powermonitor 18 can be intra-cavity (i.e., in the laser cavity 24) and/orafter the output coupler band-reflect grating optical filter 140. In onecase, having the power monitor 18 after the output coupler band-reflectgrating optical filter 140 but prior to any other integrated systemcomponents is the better implementation (but is not a necessity). Thepower monitor 18 can be a normal detector that is butt coupled to asmall tap, e.g., 1% directional coupler, from the output waveguide 20,and/or an inline power detector such as a lateral silicon PIN diode thatcollects photogenerated carriers from defect state absorption.

FIG. 2 illustrates the single frequency cavity diagram of the laser onthe silicon chip 100 according to another embodiment. The singlefrequency cavity diagram of the laser on the silicon chip 100 in FIG. 2is identical to FIG. 1 except that the passive intra-cavity opticalphase compensation characteristic of the band-reflect grating withpassive phase compensation 140 is omitted such that the laser outputcoupler grating is defined by a standard band-reflect grating 150 withall other reflectance characteristics shared with element 140.

As noted above in FIG. 1, the silicon chip 100 in FIG. 2 includes theintra-cavity transmission-mode optical band-pass filter 120 with afull-width half-maximum (FWHM) equal to or less than double thefree-spectral range of the cavity Fabry-Perot (F-P) resonances, theactive intra-cavity transmission-mode thermo-optic optical phase tunerelement 130, the output coupler band-reflect grating optical filter 140with an in-band reflectance in the range between 10% and 50% and afull-width half-maximum (FWHM) bandwidth equal to or greater than doublethe free-spectral range of the band-pass optical filter 120. The laserbeam (output) of the laser system on the silicon chip 100 is monitoredby a power monitor 18. Since the passive intra-cavity optical phasecompensation characteristic of the band-reflect grating with passivephase compensation 140 is omitted in FIG. 2, the silicon chip 100 inFIG. 2 has to place all of its reliance to maintain the same phase φ inthe active intra-cavity transmission-mode thermo-optic optical phasetuner element 130, which means that more power is required to maintainthe phase of the laser beam.

A transmission function is the product of the intra-cavitytransmission-mode optical band-pass filter 120 and Fabry-Perot (F-P)cavity. The transmission function is formally the amplitude and phasecharacteristic for various optical frequencies of a single output modegiven a unity amplitude and phase input mode. Alternatively, thetransmission function can be defined as the Fourier transform of thetransient impulse response of the optical system for the various inputand output modes of interest.

For single-mode operation with good side-mode suppression ratio, FWHM ofthe intra-cavity transmission-mode optical band-pass filter 120 shouldbe less than the Fabry-Perot (F-P) free spectral range (FSR). A lowerratio (between the intra-cavity transmission-mode optical band-passfilter 120 and Fabry-Perot free spectral range (FSR)) is better. Thefree spectral range (FSR) is the spacing in optical frequency orwavelength between two successive reflected or transmitted opticalintensity maxima or minima of an interferometer or diffractive opticalelement.

In traditional tunable lasers, the cavity length is adjusted whilemoving the intra-cavity filter wavelength (such as by turning adiffraction grating) to match the F-P and filter mode. Failure tosynchronously adjust the two (the cavity length and the intra-cavityfilter wavelength) results in mode-hopping or multi-mode operation.

As noted herein, temperature changes cause the wavelength/phase of thelaser beam to change. In accordance with embodiments, temperatureinsensitive laser operation is provided by cavity design and/or activecontrol. The intra-cavity transmission-mode optical band-pass filter 120wavelength is held constant throughout operation (outside of laserstartup initialization) through active control (active intra-cavitytransmission-mode thermo-optic optical phase tuner element 130) and/orathermal design (the passive intra-cavity optical phase compensationcharacteristic of the band-reflect grating with passive phasecompensation 140). The peak transmission angular frequency of theintra-cavity filter, ω_(f), is then considered to be constant throughoutoperation and independent of ambient temperature. The lasing mode of theFabry-Perot cavity (i.e., laser cavity 24) is locked to the intra-cavitytransmission-mode optical band-pass filter 120 through active control ofthe intra-cavity phase within the compensated round-trip phase rangeacross temperature by maximizing output power as measured byintra-cavity or extra-cavity optical power monitor 18. This can beunderstood by considering the transmission ratio, TR, between theoptical band-pass filter transmissions of intra-cavity filter at theresonance angular frequency, T_(filter)(ω_(f)), and at the angularfrequency of the operating laser mode, T_(filter)(ω_(m)):

${T\; R} = \frac{T_{filter}( \omega_{f} )}{T_{filter}( \omega_{m} )}$

Since any value of TR greater than 1 results in a reduction in the laseroutput power relative to the case where ω_(m)=ω_(f), which is thedesired operating condition for a stabilized laser operating frequencythat is temperature independent. Ensuring a less than 1:1 ratio of thefilter FWHM (in the intra-cavity transmission-mode optical band-passfilter) to Fabry-Perot mode spacing (FSR) guarantees a strong outputpower dependent error-signal for robust control of intra-cavity phase.This condition also ensures that the TR of ω_(m) is always less than theTR for ω_(m−1) or ω_(m+1) for TR<2, ensuring that the feedback loop hasa sufficiently large error signal to continuously control operation in asingle longitudinal mode of the laser cavity over the operatingtemperature range. Based on monitoring the power monitor 18, the activeintra-cavity transmission-mode thermo-optic optical phase tuner element130 adjusts the phase of the light in the laser cavity 24 and thereforecontrols ω_(m) as described by equation 53. The lasing wavelength canthen be maintained throughout the temperature range of operation withoutundergoing changes of the Fabry-Perot mode order (i.e., without modehops) to maintain error-free link operation.

Note that sub-headings are provided below for ease of understanding andnot limitation.

Passive Thermo-Optic Phase Compensation

FIG. 3A illustrates a graph 300 showing how the III-V gain region (ofthe III-V chip 10) changes the phase of the laser beam with a change inoperating temperature. As a result of an increase in temperature on thesubstrate 30, there is an increase in phase (i.e., phase change) inIII-V gain region waveform 302 and an increase in phase (i.e., phasechange in a state-of-the-art silicon passive cavity result in a waveform305. Both waveforms 302 and 305 show an increase in phase with anincrease in temperature of the silicon chip 100. However, the passivephase compensation waveform 310 of the passive intra-cavity opticalphase compensation characteristic of the band-reflect grating withpassive phase compensation 140 is passively configured to compensate forthe phase change by the III-V gain region (of the III-V chip 10) andcompensates for the resulting typical silicon passive cavity 305 phasechange. A state-of-the-art system would require active phase control(i.e., outside power) to compensate for the increase in phase shown inFIG. 3A but FIG. 1 does not (necessarily) require active phase controlalthough active intra-cavity transmission-mode thermo-optic opticalphase tuner element 130 can be (optionally) utilized. Even when activeintra-cavity transmission-mode thermo-optic optical phase tuner element130 is utilized in the silicon chip 100 in FIG. 1, less power isrequired by the active intra-cavity transmission-mode thermo-opticoptical phase tuner element 130 because the passive intra-cavity opticalphase compensation characteristic of the band-reflect grating withpassive phase compensation 140 compensates for the phase change.

FIG. 3B illustrates a schematic of the passive intra-cavity opticalphase compensation characteristic of the band-reflect grating withpassive phase compensation 140 with details of the passive thermo-opticphase compensation according to embodiments. In one case, the passiveintra-cavity optical phase compensation characteristic of theband-reflect grating with passive phase compensation 140 may be adistributed reflector grating that has smaller pitch on the left side tocompensate for the increase in phase (of the light) corresponding to theincrease in temperature of the silicon chip 100. The left side of theband-reflect grating with passive phase compensation 140 is closer tothe III-V chip 10 and pointed to the III-V chip 10, while the right sideis further away from the III-V chip 10. The pitch (linearly and/orgradually) increases from left to right (smaller pitch to wider pitch),such that the wider pitch on the right side compensates for the decreasein phase corresponding to the decrease in temperature (of the siliconchip 100). Accordingly, as the temperature increases and/or decreases(within the operation temperature (e.g., 0-85° C.) on the substrate 30of the silicon chip 100, there is a corresponding pitch variation (fromsmall pitch through wide pitch) to match the change in phase/wavelengthin the passive intra-cavity optical phase compensation characteristic ofthe band-reflect grating with passive phase compensation 140. Thesmaller pitch on the left side reflects the light with the hightemperature (higher phase and smaller wavelength), while the wider pitchon the right side reflects the light with the lower temperature (lowerphase and longer wavelength).

Intra-Cavity Filters

FIG. 4 illustrates an example intra-cavity transmission-mode opticalband-pass filter 120 according to an embodiment. In one implementation,the intra-cavity transmission-mode optical band-pass filter 120 may beany configuration of a ring-resonator, such as a Mach-Zehnder and/orgrating transmission-mode filter with appropriate free-spectral range(FSR) and bandwidth suitable for laser cavity construction. In thisimplementation, FIG. 4 shows the ring-resonator with connected waveguide20 for input and output of the light. The ring-resonator has a heater(e.g., a resistor or resistive element) that receives power in order tocontrol the temperature of the ring-resonator. The filter resonancefrequency of the ring-resonator is to be maintained throughout laseroperation. In the example intra-cavity transmission-mode opticalband-pass filter 120, FIG. 4 shows a front-up approach which is afirst-order ring resonator filter that is thermally controlled (thuscontrolling the ring resonance frequency) to a constant temperatureabove the maximum designed operation ambient temperature of the lasersystem in the silicon chip 100.

A ring-resonator, also referred to as an optical ring resonator, is aset of waveguides in which at least one is a closed loop coupled to somesort of light input and output. These can be, but are not limited tobeing, waveguides. The concepts behind optical ring resonators use lightand obey the properties behind constructive interference and totalinternal reflection. When light of the resonant wavelength/frequency ispassed through the loop from input waveguide, the light builds up inintensity over multiple round-trips due to constructive interference andis output to the output bus waveguide which serves as a detectorwaveguide. Because only a select few wavelengths will be at resonancewithin the loop, the optical ring resonator functions as a filter.Additionally, two or more ring waveguides can be coupled to each otherto form an add/drop optical filter.

Active Round-Trip Phase Control

FIGS. 5A and 5B show two different examples of the active intra-cavitytransmission-mode thermo-optic optical phase tuner element 130 accordingto an embodiment. Although examples are provided, any method of phasecontrol is suitable if the total range is approximately (˜) 4π.

FIG. 5A illustrates an implementation (which may be preferred but is nota necessity) of the active intra-cavity transmission-mode thermo-opticoptical phase tuner element 130 as a broadband thermo-optic tunerbecause the zero amplitude response and ease of control of broadbandthermo-optic tuner. The broadband thermo-optic tuner has a waveguide 20in which the light travels in and out, and a heater 505. Current can beapplied to the heater 505 to actively control the round-trip phase.

FIG. 5B illustrates another implementation of the active intra-cavitytransmission-mode thermo-optic optical phase tuner element 130 asnarrowband thermo-optic tuners (ring-resonator all-pass filters). Thenarrowband thermo-optic tuners are suitable as well, but add complexityfor resonance frequency control. The narrowband thermo-optic tuners showa waveguide 20 with two ring-resonators 510, and each ring-resonator hasa heater 505 to control the bandwidth.

Note also that carrier-injection tuners can be utilized either in thesilicon cavity and/or in the III-V die, but the carrier-injection tuneradds complexity of amplitude fluctuations.

Reflectors

FIG. 6 illustrates an example output coupler band-reflect gratingoptical filter 140 according to an embodiment. In one implementation,the output coupler band-reflect grating optical filter 140 may be astandard sidewall grating (partially etched or fully etched), which is asuitable output coupler for basic laser operation with in-bandreflectance between 10% and 50%. The in-band reflectance of the sidewallgrating is determined by III-V current-gain characteristic (of the III-Vchip 10), passive cavity loss, coupling efficiency, and output power.

The bandwidth of the sidewall grating can be designed to have at least a1 decibel (dB) suppressed reflectance for non-lasing filter order peaks(e.g., the peaks may be greater than (>) 3 dB in one case) inconfigurations without a compound intra-cavity filter characteristicthat otherwise suppresses alternate filter order transmittance. Usingthe sidewall grating, compensation of the round-trip cavity phase withreduction in effective cavity length as a function of increasingtemperature is included.

Mode Converter

FIG. 7 illustrates an example of the mode converter 16 according toembodiment. Any standard laser to silicon-on-insulator (SOI) waveguidemode coupler and packaging strategy can be applied. Requirements ofefficiency and reflectance are tied together in a requirement for stablesingle-mode operation. Ideal reflectance of 1e⁻⁵ (achievable throughangled facet interface) may be relaxed to as high as 1% in highlyefficient coupling schemes that allow the III-V die 10 HR back coatingfacet 12 and the output coupler band-reflect grating optical filter 140(e.g., output coupler grating) to dominate the cavity Fabry-Perotcharacteristic.

Power Monitors

FIG. 8 illustrates an example implementation of the power monitoraccording to an embodiment. The power monitor 18 may have an N+ dopedsilicon contact connected to a P+ doped silicon contact. The waveguide20 connects laterally through the power monitor 18. Photocurrentproportional to the waveguide power (i.e., light beam) is generated inthe power monitor 18, and the photocurrent flow perpendicular to thewaveguide 20.

Power monitoring by the power monitor 18 in the laser system isimportant for control of the intra-cavity phase to maintain efficientsingle-mode operation for error-free link operation using the proposedlaser across operating temperatures.

The power monitor 18 can be intra-cavity and/or after the output couplerband-reflect grating optical filter 140 (i.e., output coupler grating).Positioning the power monitor 18 after the output coupler band-reflectgrating optical filter 140 but prior to any other integrated systemcomponents may be the better implementation (but is not a necessity).

The power monitor 18 can be a normal detector that is butt coupled to asmall tap, e.g., 1% directional coupler, from the output waveguide 20,and/or an inline power detector such as a lateral silicon PIN diode thatcollects photogenerated carriers from defect state absorption.

Now, moving away from the sub-headings, multi-wavelength operation isdiscussed in FIGS. 9 and 10. Multi-wavelength operation may employN-port demultiplexer filters, such as Mach-Zehnder interferometer (MZI)CWDM filters developed by MZI for transceiver applications.

FIG. 9 illustrates a multi-frequency diagram of a laser on the siliconchip 100 according to an embodiment. In FIG. 9, the silicon chip 100 nowincludes a coarse N-port wavelength division multiplexing (WDM)demultiplexing filter 905 positioned (directly) after the mode converter16. The input port (IN) of the coarse N-port WDM demultiplexing filter905 connects to the mode converter 16 to receive the light, and theoutput ports (OUT) of the coarse N-port WDM demultiplexing filter 905connect to their respective 1-N integrated photonic circuits 25. As canbe seen there are multiple output port. For example, the coarse N-portWDM demultiplexing filter 905 may receive light at different wavelengthsat the input side from the mode converter 16, such that the coarseN-port WDM demultiplexing filter 905 demultiplexes (separates) the lightby wavelength and outputs light of each wavelength to an individualoutput port. The output ports are connected to the circuits 25, and the1-N circuits 25 each include the intra-cavity transmission-mode opticalband-pass filter 120, the active intra-cavity transmission-modethermo-optic optical phase tuner element 130, and the band-reflectgrating with passive phase compensation element 140. In this case,silicon chip 100 is configured to output multiple light beams with eachat a different wavelength.

Turning to FIG. 10, another multi-frequency diagram of a laser on thesilicon chip 100 according to an embodiment. FIG. 10 is similar to FIG.9 except that the intra-cavity transmission-mode optical band-passfilter 120 is omitted because the coarse N-port WDM demultiplexingfilter 905 is sufficiently narrowband to eliminate the need for aseparate optical band-pass filter 120. In FIG. 10, the coarse N-port WDMdemultiplexing filter 905 is now a narrowband N-port WDM demultiplexingfilter that meets all the previously described parameters of theintra-cavity transmission-mode optical band-pass filter 120. FIG. 10 isa multi-frequency diagram of the laser shown in FIG. 2, and the siliconchip 100 now includes the narrowband N-port wavelength divisionmultiplexing (CWDM) demultiplexing filter 905 positioned (directly)after the mode converter 16. As noted above, the input port of thenarrowband N-port WDM demultiplexing filter 905 connects to the modeconverter 16 to receive the light, and the output ports of the coarseN-port WDM demultiplexing filter 905 connect to their respective 1-Ncircuits 25. For example, the narrowband N-port WDM demultiplexingfilter 905 may receive light at different wavelengths at the input sidefrom the mode converter 16, such that the narrowband N-port WDMdemultiplexing filter 905 demultiplexes (separates) the light bywavelength and outputs light of each wavelength to an individual outputport. The output ports are connected to the circuits 25, and the 1-Ncircuits 25 each include the active intra-cavity transmission-modethermo-optic optical phase tuner element 130 and the output couplerband-reflect grating optical filter 140. In this case, silicon chip 100is configured to output multiple light beams with each at a differentwavelength. Besides having the narrowband N-port WDM demultiplexingfilter 905 included and removing the intra-cavity transmission-modeoptical band-pass filter 120, the silicon chip 100 in FIG. 10 operatesas discussed in FIG. 1.

FIG. 11 illustrates a method 1100 of configuring the semiconductor chip100 according to an embodiment. Reference can be made to FIGS. 1, 2, 9,and 10. At block 1105, the optical gain chip 10 is attached to asemiconductor substrate 30.

At block 1110, the integrated photonic circuit 25 is provided on thesemiconductor substrate 30, and the optical gain chip 10 opticallycoupled to the integrated photonic circuit 25 forms the laser cavity 24.

At block 1115, the integrated photonic circuit 25 comprises the outputcoupler band-reflect optical grating filter with passive phasecompensation 140, the active intra-cavity thermo-optic optical phasetuner element 130, and the intra-cavity optical band-pass filter 120.

At block 1120, the output coupler band-reflect optical grating filterwith passive phase compensation 140, the active intra-cavitythermo-optic optical phase tuner element 130, and the intra-cavityoptical band-pass filter 120 are optically coupled together.

The mode converter 16 is coupled between the optical gain chip 10 andthe integrated photonic circuit 25. The output coupler band-reflectoptical grating filter with passive phase compensation 140 is configuredto reduce a net round trip phase change to within 4π over a temperaturerange. The temperature range is 0-85° Celsius.

The output coupler band-reflect optical grating filter with passivephase compensation 140 comprises a distributed reflector grating element(e.g., as shown in FIG. 3). The distributed reflector grating elementhas a smaller pitch at a first end and a wider pitch at the second end.The distributed reflector grating element is configured to shorten aneffective cavity (length) of the laser cavity 24 with increasingtemperature through increased index contract. The distributed reflectorgrating element has an elongated direction (e.g., length) and a widthdirection. The distributed reflector grating element changes in pitchalong the elongated direction (e.g., length) such that the distributedreflector grating element varies from the smaller pitch at the first endand increases to the wider pitch at the second end.

When the N-port demultiplexing filter 905 is included the silicon chip100, the silicon chip 100 includes output coupler band-reflect opticalgrating filter with passive phase compensation 140 (as shown in FIGS. 9and 10). Although FIGS. 9 and 10 show different implementations. TheN-port demultiplexing filter 905 is configured to provide differentwavelengths of light to individual ones of the plurality of (1-N)integrated photonic circuits 25. In FIGS. 9 and 10, the mode converter16 is coupled between the optical gain chip 10 and the coarse N-port WDMdemultiplexing filter 905.

It will be noted that various semiconductor device fabrication methodsmay be utilized to fabricate the components/elements discussed herein asunderstood by one skilled in the art. In semiconductor devicefabrication, the various processing steps fall into four generalcategories: deposition, removal, patterning, and modification ofelectrical properties.

Deposition is any process that grows, coats, or otherwise transfers amaterial onto the wafer. Available technologies include physical vapordeposition (PVD), chemical vapor deposition (CVD), electrochemicaldeposition (ECD), molecular beam epitaxy (MBE) and more recently, atomiclayer deposition (ALD) among others.

Removal is any process that removes material from the wafer: examplesinclude etch processes (either wet or dry), and chemical-mechanicalplanarization (CMP), etc.

Patterning is the shaping or altering of deposited materials, and isgenerally referred to as lithography. For example, in conventionallithography, the wafer is coated with a chemical called a photoresist;then, a machine called a stepper focuses, aligns, and moves a mask,exposing select portions of the wafer below to short wavelength light;the exposed regions are washed away by a developer solution. Afteretching or other processing, the remaining photoresist is removed.Patterning also includes electron-beam lithography.

Modification of electrical properties may include doping, such as dopingtransistor sources and drains, generally by diffusion and/or by ionimplantation. These doping processes are followed by furnace annealingor by rapid thermal annealing (RTA). Annealing serves to activate theimplanted dopants.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

What is claimed is:
 1. A semiconductor chip, comprising: an optical gainchip attached to a semiconductor substrate; and an integrated photoniccircuit on the semiconductor substrate, the optical gain chip opticallycoupled to the integrated photonic circuit thereby forming a lasercavity, wherein the integrated photonic circuit comprises: an activeintra-cavity thermo-optic optical phase tuner element; an intra-cavityoptical band-pass filter; and an output coupler band-reflect opticalgrating filter with passive phase compensation; wherein the activeintra-cavity thermo-optic optical phase tuner element, the intra-cavityoptical band-pass filter, and the output coupler band-reflect opticalgrating filter with passive phase compensation are optically coupledtogether.
 2. The semiconductor chip of claim 1, further comprising amode converter coupled between the optical gain chip and the integratedphotonic circuit.
 3. The semiconductor chip of claim 1, wherein theoutput coupler band-reflect optical grating filter with passive phasecompensation is configured to reduce a net round trip phase change towithin 4π over a temperature range.
 4. The semiconductor chip of claim3, wherein the temperature range is 0-85° Celsius.
 5. The semiconductorchip of claim 1, wherein the output coupler band-reflect optical gratingfilter with passive phase compensation comprises a distributed reflectorgrating element.
 6. The semiconductor chip of claim 5, wherein thedistributed reflector grating element has a smaller pitch at a first endand a wider pitch at a second end; wherein the distributed reflectorgrating element is configured to shorten an effective cavity of thelaser cavity with increasing temperature through increased indexcontract.
 7. The semiconductor chip of claim 6, wherein the distributedreflector grating element has an elongated direction and a widthdirection; wherein the distributed reflector grating element changes inpitch along the elongated direction such that the distributed reflectorgrating element varies from the smaller pitch at the first end andincreases to the wider pitch at the second end.
 8. A semiconductor chip,comprising: an optical gain chip attached to a semiconductor substrate;an N-port demultiplexing filter; and a plurality of integrated photoniccircuits on the semiconductor substrate, the optical gain chip opticallycoupled to the plurality of integrated photonic circuits thereby forminga laser cavity, wherein the plurality of integrated photonic circuitseach comprise: an output coupler band-reflect optical grating filterwith passive phase compensation; an active intra-cavity thermo-opticoptical phase tuner element; and an intra-cavity optical band-passfilter; wherein the output coupler band-reflect optical grating filterwith passive phase compensation, the active intra-cavity thermo-opticoptical phase tuner element, the intra-cavity optical band-pass filterare optically coupled together; and wherein the N-port demultiplexingfilter is configured to provide different wavelengths of light toindividual ones of the plurality of integrated photonic circuits.
 9. Thesemiconductor chip of claim 8, further comprising a mode convertercoupled between the optical gain chip and the N-port demultiplexingfilter.
 10. The semiconductor chip of claim 8, wherein the outputcoupler band-reflect optical grating filter with passive phasecompensation is configured to reduce a net round trip phase change towithin 4π over a temperature range.
 11. The semiconductor chip of claim10, wherein the temperature range is 0-85° Celsius.
 12. Thesemiconductor chip of claim 8, wherein the output coupler band-reflectoptical grating filter with passive phase compensation comprises adistributed reflector grating element.
 13. The semiconductor chip ofclaim 12, wherein the distributed reflector grating element has asmaller pitch at a first end and a wider pitch at a second end; whereinthe distributed reflector grating element is configured to shorten aneffective cavity of the laser cavity with increasing temperature throughincreased index contract.